1. Field of the Invention
The present invention relates to phase shift masks, and more particularly to a phase shift mask of a spatial frequency-modulated type and a method of making the same.
2. Description of the Prior Art
Presently, there is a requirement of masks for achieving hyperfine patterning of a submicron-grade, according to the trend of high integration of semiconductor devices. Phase shift masks have been developed as a technique for satisfying this requirement. The principle of such shift masks will now be described.
For manufacturing a phase shift mask, phase shift film 1 basically is necessary (see FIG. 1). The function of phase shift film 1 is to phase-shift the incident light, illustrated by the downward pointing arrows in FIG. 1. In FIG. 1, reference numeral 2 denotes a chromium layer as a light shield layer and reference numeral 3 denotes a light transmitting substrate made of, for example, quartz.
Referring to FIG. 2, graphs are illustrated that represent the amplitude of light through the mask. Graph "a" illustrates the amplitude of light with phase shift film 1 not present, whereas graph "b" illustrates the amplitude of light with phase shift film 1 present.
As illustrated in FIG. 2, the amplitude of light is phase-shifted by 180.degree. by phase shift film 1.
Here, if the refractive index of phase shift film 1 is represented by n, and the thickness of phase shift film 1 is represented by d, and the refractive index of air is represented by n.omicron., the phase difference .delta. between graphs "a" and "b" of FIG. 2 can be expressed by the following equation: ##EQU1##
In accordance with formula (1), phase difference .delta. should be 180.degree. in order for the phase to be ideally shifted. If .pi. is substituted into formula (1) instead of the phase difference .delta., the thickness d of phase shift film 1 to be appropriately shifted can be put into the following formula (2): ##EQU2##
Now, a comparison of the above-mentioned phase shift mask with a general mask will be made with reference to FIGS. 3a through 4d.
FIG. 3a illustrates general pattern mask 4 disposed over substrate 5 in a parallel-aligned manner. FIG. 3b illustrates the amplitude of light out of mask 4. FIG. 3c illustrates the amplitude of light out of substrate 5. FIG. 3d illustrates the intensity of light out of substrate 5.
As shown in FIG. 3b, the amplitude of light out of adjacent openings of mask 4 constructively interfere with each other, thereby causing the amplitude and intensity differences of the transmitted light to be decreased as shown in FIGS. 3c and 3d. Such reduced amplitude and intensity differences result in an unclear distinction between dark portions and bright portions out of substrate 5.
Since the above effect is increased with hyperfine patterning, it is impossible to achieve submicron-grade patterning by using a general pattern mask such as mask 4.
FIG. 4a illustrates phase shift mask 7 having phase shift film 6 disposed between patterns of mask 4 in an aligned manner. FIG. 4b illustrates the amplitude of light out of the areas between the patterns of mask 4. FIG. 4c illustrates the amplitude of light out of substrate 5. FIG. 4d illustrates the intensity of light out of substrate 5.
In this case, the amplitude and intensity differences of the light are increased, as illustrated in FIGS. 4c and 4d. Accordingly, the distinction between dark portions and bright portions out of substrate 5 becomes more defined, so that hyperfine patterning may be advantageously performed.
Such phase shift masks may be classified as spatial frequency-modulated type, edge-emphasized type and shield effect-emphasized type, and these types of phase shift masks will be described with reference to FIGS. 5a to 5f.
FIG. 5a illustrates a spatial frequency-modulated type phase shift mask. The phase shift mask is made by forming a chromium film over quartz substrate 8, patterning the chromium film to form patterned chromium film 9, and forming phase shift film 10 between adjacent portions of patterned chromium film 9.
On the other hand, FIGS. 5b and 5c illustrate an edge-emphasized type phase shift mask. In particular, FIG. 5b illustrates a structure in which each portion of patterned chromium film 9 is surrounded by phase shift film 10, whereas FIG. 5c illustrates a structure in which phase shift film 10 is disposed over each portion of patterned chromium film 9.
FIGS. 5d and 5f illustrate a shield effect-emphasized type phase shift mask. In particular, FIG. 5d illustrates a structure in which phase shift film 10 is formed between adjacent patterned portions of chromium film 9. FIG. 5e illustrates a structure in which phase shift film 10 is disposed over and between two portions of chromium film 9, with chromium film 9 having been patterned and separated as shown in FIG. 5e. On the other hand, FIG. 5f illustrates a structure in which chromium film 9 is formed on portions of quartz substrate 8, which has been patterned and repeatedly etched to the predetermined depth to provide appropriate phase shifting as shown in FIG. 5f. The spatial frequency-modulated type phase shift masks, to which the present invention relates, will be described with reference to FIGS. 6a to 6g.
First, over glass substrate 11 is coated chromium layer 12, having a thickness of about 1,000 .ANG. to 1,500 .ANG., and photoresist film 13, in this order, as shown in FIG. 6a.
Thereafter, photoresist film 13 is patterned to form a plurality of uniformly spaced photoresist patterns 13a, as shown in FIG. 6b.
Using photoresist patterns 13a as a mask, chromium layer 12 is subjected to an etching to form a plurality of uniformly spaced chromium patterns 12a, as shown in FIG. 6c. Subsequently, photoresist patterns 13a are removed.
Over the resultant entire exposed surface, light transmitting film 14 for defining a phase shift region is coated to have a predetermined thickness, as shown in FIG. 6d. Light transmitting film 14 may be made of silicon oxide, silicon-on-glass (SOG), or a polymer material.
Photoresist 15 is then deposited over light transmitting film 14, as shown in FIG. 6e.
As shown in FIG. 6f, photoresist 15 is selectively subjected to an exposure utilizing electron beams and then developed so that photoresist pattern 15a remains over a region defined as the phase shift region.
Using photoresist pattern 15a as a mask, light transmitting film 14 is then selectively subjected to an etching process to form phase shift region 14a, as shown in FIG. 6g. Thereafter, photoresist pattern 15a is removed.
Now, an effect of the conventional spatial frequency-modulated type phase shift mask manufactured as mentioned above will be described with reference to FIGS. 7a to 7c.
FIG. 7a is a sectional view of a phase shift mask manufactured according to a conventional technique, whereas FIGS. 7b and 7c illustrate the energy and intensity of light passing through the phase shift mask shown in FIG. 7a.
As shown in FIGS. 7b and 7c, the light passing through the phase shift region of the phase shift mask is phase-shifted as compared with the light passing through other portions of the phase shift mask at which the phase shift region is not present.
On the other hand, the intensity of light at the phase shift region is the same as that at other portions of the phase shift mask, as shown in FIG. 7c. Accordingly, the intensity of light transferring the mask shape to a wafer becomes high. That is, the phase shift angle of light is most ideal at 180.degree. and can be adjusted by the refractive index of the material comprising the phase shift region and the thickness of the phase shift region.
However, the conventional spatial frequency-modulated phase shift mask encounters the following problems.
First, since a chromium pattern is used as a light shield and selectively etched, with the phase shift region disposed between and partially over adjacent portions of the chromium pattern, the overall processes are complex, thereby causing the quality of produced masks to deteriorate.
Second, since the phase shift region is a nonconductor, a conductive film should be provided for pattern printing utilizing electron beams so as to ground easily electrons generated at the surface of mask, thus avoiding charging at the surface. As a result, the number of steps in the overall process increases due to the addition of a process for forming a conductive film. Furthermore, when the conductive film is contaminated, regular light transmission may be impeded.
Third, it is difficult to adjust the thickness of light-transmitting phase shift region 14a and to provide a surface smoothness, since phase shift region 14a is deposited over chromium layer 12 as the light shield layer having a thickness of 1,000 .ANG. to 1,500 .ANG., and a step may be formed between the glass substrate and the chromium layer, as illustrated in FIG. 8.
Fourth, there is a problem in etching the phase shift region material, since the phase shift region may be of a glass material which is coated over the substrate which may be of the same glass material, and then dry etched.